Not applicable.
1. Field of Invention
The present invention relates to measurement of viscosity with a variable speed drive and force-sensing device.
2. Description of Prior Art
A liquid between two surfaces will shear when one surface moves relative to the other. The force needed to make such a movement is directly related to the viscosity of the liquid (with the mechanical configuration factored out). Viscometers typically rotate a cup or bob within a still cylinder with the liquid therebetween, or rotate an outer cylinder while keeping the inside coaxial bob inert. In such examples, torque is directly related to the viscosity of the liquid (again with mechanical configuration factored out).
Several types of arrangement have been applied to measure the torque due to the viscosity of the liquid. In U.S. Pat. No. 3,435,666, a spring is attached to the inside bob while driving the outer cylinder. The mechanical dial reading reflects the deflection of the spring, which in turn is proportional to the torque applied by the liquid. However, it is relatively expensive to convert the deflection of the spring to electronic signal for automatic data acquisition and control purpose. In U.S. Pat. No. 5,503,003, a spring connection between two slotted wheels, where one wheel is attached to a mechanism that is sensitive to the drag caused by a viscous liquid, and the other wheel is attached to a mechanical drive assembly. When operated, the wheel sensitive to drag deflects with respect to the driven wheel. Optical sensors detect the resulting deflection that is calibrated to indicate the viscosity of the liquid. The drawback of this arrangement is that the response time of the spring connection is relatively long and the spring connection is prone to overload damage.
It is an object of this invention to provide a reliable, but rugged and economical instrument with integrated electronics usable in viscosity measuring applications, under atmospheric, pressurized, lowland high temperature conditions.
It is another object to provide a viscometer that operates with a wide range of liquids with an extremely fast response.
It is another object of the invention to provide a viscometer that is economical to manufacture yet meets industry standards of accuracy, reliability, durability, dependability, and ease of maintenance and cleaning.
The present invention provides a new and improved apparatus and method for measuring the viscosity of a fluid. The apparatus and method of the present invention are particularly useful for measuring the instant shear stress and viscosity of a liquid with viscoelasticity property, where a fast response time of measurement is essential. The apparatus is also particularly useful for accurately converting the torque applied by the liquid to electronic signals very economically. The present invention provides an apparatus and method for making fluid viscosity measurements employing an arm and an electronic force sensor.
A viscometer in accord with the present invention conveniently comprises a stationary frame from which a rotatable sleeve is suspended and includes a means for rotating the sleeve. Suspended within the sleeve is a bob capable of angular motion about the longitudinal axis of the sleeve. The device is constructed so that the bob and at least the portion of the sleeve near the bob may be immersed within the liquid, the viscosity of which is to be determined. The bob is suspended from the stationary frame by a low friction bearing which permits limited angular motion about its center of rotation. An arm is attached to the bob shaft or extended portion of the bob, and the arm is contacting an electronic force sensor. Given the known characteristics of the viscometer, the force applied on the force sensor is proportional to the viscosity of the liquid.
Alternative embodiments of the present method comprise a still sleeve, and rotate the bob through a force sensor and an arm.
The apparatus and method of the present invention provide a fast response, bi-directional, and economical way to measure the shear stress property of fluid under shear condition.